Defibrillation apparatus and method

ABSTRACT

An implanted defibrillator continuously monitors a patient&#39;s heart to detect the presence of fibrillation and repeatedly, automatically computes the approximate entropy of a series of data representing the fibrillating heart at a moment in time. The first approximate entropy score that meets a predetermined relation with respect to a predetermined threshold value activates an energy delivery system to defibrillate the heart with a low level shock. The process continues until defibrillation is successful. An external defibrillator incorporates the approximate entropy algorithm to achieve low level defibrillation. A method is disclosed which times the delivery of a defibrillating shock to a fibrillating heart to coincide with the moment that the defibrillation threshold is at a minimum.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the treatment of heart failures anddisorders and, more particularly, to apparatus and methods fordefibrillating the heart through selective application of energy to theheart.

2. Description of the Related Art

Individuals that have congenital heart disease, particularly, anarrhythmia, or who have previously suffered one or more episodes ofheart failure or heart disorder, are at a significantly higher risk thanthe general population of experiencing a future episode of heart failureor heart disorder. Two well known types of arrhythmias that afflictindividuals are ventricular fibrillation ("VF") and atrial fibrillation("AF"). VF constitutes the primary mechanism seen in sudden cardiacarrest and is manifested by the absence of organized electrical activityand synchronized mechanical pumping. AF is manifested by the extremelyrapid contractions of the atria. Both arrhythmias are serious heartdisorders but VF is particularly lethal if not quickly treated.

Electrical defibrillation is a known method for bringing a patient outof VF or AF. It typically entails the application of an electric shockof substantial energy, e.g. 10-30 joules or more, across a portion ofthe myocardium in order to depolarize the ventricle or atrium and toreturn organized, sinus rhythm to the heart. Multiple shocks are oftenrequired. Originally, restoring normal heart function to these patientscould be accomplished only with external defibrillation treatment at ahospital, in an ambulance, or other medical care facility. Thistreatment involved the placement and activation of paddles connected toan electric shock apparatus on the patient's thorax. Thereafter, theadvent of the automatic, surgically-implanted cardioverter defibrillator("AICD" or "ICD") permitted VF and AF treatment outside a hospital,ambulance, or other medical care facility. Typically, upon sensing anoccurrence of VF or AF, an ICD automatically applies one or moredefibrillating shocks to the heart until normal sinus rhythm isrestored.

FIG. 1 depicts a representative example of a conventional ICD 20implanted in the subclavian area 8 of a patient 10 for the purpose ofdefibrillating the patient's heart 12. The ICD 20 includes an enclosedcontrol box, or enclosure 22 that houses a processor 24 and a shock orpulse generator 26. The size of the control box is relatively large whencompared to a conventional pacemaker. For example, the Ventritex"V-145D" ICD has a volume of 57 cc and weighs 109 grams; CPI's "Mini IIModel 1762" has a volume of 59 cc and weighs 115 grams; and theMedtronic "Micro Jewel 7221 Cx" has a volume of 72 cc and weighs 116grams. The conventional pacemaker, on the other hand, has a volumeconsiderably less than 50 cc. The largest component in the control boxis the energy-storing capacitor 27 of the shock generator capable ofproviding a shock of 30 or more joules. Its relatively large size is theprimary limitation in development efforts to further downsize ICD's.

In order to accomplish appropriate defibrillation, a sensing/energydelivery electrode 28 is placed endocardially within a ventricle of theheart 12. The electrode senses ventricular rate and transmits, through asubcutaneous rate sensing lead 30, information reflective of this rateto the processor located in the control box. Additionally, the electrodeis connected to the shock generator 26 in the control box 22 fordelivering shocks to the heart.

In operation, when the heart 12 starts to fibrillate, the electrode 28provides information to the processor which, in turn, activates theshock generator 26. The capacitor 27 is charged and delivers an electricshock to the heart electrode. If the processor determines that the heartis no longer fibrillating, it deactivates the shock generator 26,thereby signifying successful defibrillation. However, if the processordetermines that the heart is still in fibrillation, it reactivates theshock generator so as to recharge the capacitor via a battery (notshown) to thereby cause the generator 26 to deliver another shock to theheart 12. This process continues for a preset number of times untileither the heart is successfully converted out of fibrillation, or somepreset limit is reached (such as 5 consecutive unsuccessful shocks).

FIG. 2 shows a representative example of a conventional externaldefibrillator 60 functioning on a patient 50 lying in the proneposition, on a bed in an emergency room. Typically, an ECG (not shown)is connected to the patient to monitor the heart activity. Thedefibrillator 60 is powered by AC wall power 70 which is convertedinternally to DC power. The operator manually sets the energy level tobe delivered to the patient via an output controller 76 and depresses abutton 78 to activate the defibrillator. Therapy leads 72 transmit theenergy to a pair of therapy paddles 74 which are placed on the chest 52of the patient 50 for defibrillation.

Conventional ICD's do have advantages. For one, they tend to reduce therisk of sudden death or serious injury from arrhythmias. Thus, they havehad the capacity to dramatically increase the life expectancy ofpatients within whom they are implanted. Nevertheless, ICD's have notgained as widespread acceptance as might have been expected. A primaryreason is that after the fibrillation is sensed an ICD needs to delivera shock of a relatively strong intensity in order to successfullydefibrillate the heart. This requirement has a number of drawbacks.First, these powerful shocks, which sometimes reach thirty joules ormore, can cause severe discomfort and undue pain to the patient.Consequently, many candidates for ICD's are simply not prepared toendure the dreaded defibrillating shock, as well as the psychologicaluneasiness or fear accompanying the knowledge that a shock may occur atany time. Thus, they refuse to have a given ICD implanted. Others havefainted from the magnitude of the defibrillating shocks, thereby causingadditional health and safety concerns. As is conventionally understood,the discomfort and pain caused by the required shock is equally presentwith external defibrillation.

Second, to successfully deliver the requisite shock in a portable,battery-operated ICD device, a relatively large energy-storing capacitorneeds to be employed. This constraint tends to severely limit theability of ICD manufacturers to downsize these rather large devices.

A third problem is that the relatively high energy requirement of theICD tends to compromise the life of its battery, which is used torepetitively charge the capacitor. This is a substantial drawbackbecause access to the surgically-implanted ICD for the purpose ofbattery replacement necessitates another invasive operation on analready sick patient.

Various efforts to address the aforementioned problems have beenundertaken. Some of these efforts have centered on miniaturizing theenergy-storing capacitors. Other efforts have focused on reducing theminimum amount of energy needed to successfully convert VF to normalsinus rhythm, known as the defibrillation threshold. These effortsprimarily involve varying the shock waveforms. For example, one approachis to change the shock delivery from a monophasic waveform to a biphasic(bidirectional) waveform. Another is to change the shape, or "tilt," ofthe biphasic current delivery over time. Still another approach is thedelivery of pulses simultaneously over two current pathways. However, todate, each of these efforts have only modestly reduced the requiredconversion energy. Above all cardiac defibrillation algorithms employedin defibrillators do not adequately take into account the underlyingmechanisms of fibrillation.

Additionally, conventional defibrillators work on the theory that theabnormal activation patterns in a fibrillating heart takes a completelyrandom and unintelligible form. Consequently, defibrillation researchhas focused on (1) identifying the existence of fibrillation, and then(2) generating and applying various types of pulses (shocks) to theheart to terminate it. While such efforts are commendable, they have notbeen adequately dispensed toward effectively addressing the aboveproblems.

Accordingly, it should be appreciated that there exists a definite needfor a defibrillation apparatus and method that can successfullydefibrillate a heart in VF or AF with much lower energy levels than canbe presently achieved. There also exists a definite need for adefibrillator which is smaller and lighter than existing devices.Further, there is a definite need for a defibrillation method,manifested in either an ICD or external defibrillator, which tends tomore successfully defibrillate a patient and at the same time tominimize the pain perceived by the patient.

SUMMARY

The present invention, which addresses these needs, is embodied in anapparatus and method which tends to reduce the amount of energy thatwould otherwise be necessary to defibrillate a heart by selectivelyproviding the energy to the heart based upon the approximate entropy("ApEn") of the heart. When the ApEn of a given heart has apredetermined relation with respect to a preset value, defibrillationcan be advantageously achieved at a reduced energy level. In oneapplication, this level is less than 5 joules; in another it is lessthan 10 joules.

In particular, the apparatus includes an energy delivery system fordelivering to a heart an electric shock of a predetermined energy leveland a controller that receives data indicative of the rhythm of theheart and that selectively activates the energy delivery system inresponse to an approximate entropy score derived by the controller fromthe data. The invention has utility for improved defibrillation of afibrillating ventricle or atrium of the heart. Further, the invention isembodied in both an implantable defibrillator and externaldefibrillator.

In another aspect of the invention, the apparatus includes a sensor,which is associated with the heart and is configured to detect a heartrhythm and transmit data indicative of the rhythm, an energy deliverysystem associated with the heart for delivery to the heart an electricshock of a predetermined energy level, and a controller responsive tothe sensor that selectively activates the energy delivery system inresponse to an approximate entropy score derived by the controller fromthe data. Further, the apparatus may include multiple sensors or onesensor having multiple sensing poles. Moreover, the energy deliverysystem has an electric shock generator and at least one therapy leadassociated with the heart. The sensor or sensors and the therapy lead orleads can also be integral with one another.

In a more detailed feature of the invention, the controller includessensor data and approximate entropy processors that together cooperatewith a comparator so as to permit an energy delivery system actuator toactivate the energy delivery system. Specifically, the data processorreceives heart data and repeatedly computes a plurality of data pointssubstantially equally spaced in time. The approximate entropy processorthen receives the data points and repeatedly computes an ApEn score fromthe data points. The comparator then compares the ApEn score to at leastone preset ApEn threshold value stored in a memory of the controller,and provides output that is monitored by the energy delivery systemactivator to monitor the output of the comparator. In monitoring theoutput of the comparator, the activator activates the energy deliverysystem when the ApEn score has at least one predetermined relation withrespect to the at least one preset ApEn threshold value, therebydefibrillating the heart.

In another more detailed feature of the invention, the controllerrepeatedly computes from the sensor data a scalar times series which iscomprised of discrete data point equally-spaced in time. The controllerthen derives the ApEn score from the scalar time series. The ApEn scoreis defined by the equation:

    ApEn(m,r,N)=φ.sup.m (r)-φ.sup.m+1 (r)

and ranges from 0 to 2. The controller compares the ApEn score to afirst preset approximate entropy threshold value stored in a memory ofthe controller, and then selectively activates the energy deliverysystem when the ApEn score has a predetermined relation with respect toa first preset approximate entropy threshold value. In one application,this predetermined relation arises when the ApEn score is less than thefirst preset ApEn threshold value. In another application, this relationarises when the ApEn score is greater than the first preset ApEnthreshold value. In yet another application, this predetermined relationarises when the ApEn is either less than the first preset thresholdvalue or greater than a second preset threshold value.

Other features and advantages of the present invention should becomemore apparent from the following description of the preferredembodiments, taken in conjunction with the accompanying drawings, whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a conventional ICD implanted in apatient.

FIG. 2 is a perspective view of a conventional external defibrillatorattached to a patient in the recumbent position.

FIG. 3a is a block diagram of a defibrillator interconnected to a heartshown in perspective view.

FIG. 3b is a block diagram of the controller of the defibrillator shownin FIG. 3a.

FIG. 4 is a flowchart illustrating steps performed in the preferredmethod of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention summarized above and defined by the enumerated claims maybe better understood by referring to the following detailed description,which should be read in conjunction with the accompanying drawings. Thisdetailed description of particular preferred embodiments, set out belowto enable one to build and use those particular implementations of theinvention, is not intended to limit the enumerated claims, but to serveas particular examples thereof. The particular examples set out beloware the preferred specific implementations of: (1) an improved automaticimplanted cardioverter defibrillator, namely, one that automaticallydefibrillates a heart in either VF or AF based upon an approximateentropy algorithm; and (2) an improved external defibrillator thatoperates on a heart in VF or AF based upon the approximate entropyalgorithm. The invention, however, may also be applied to other types ofarrhythmias.

Before describing the invention in further detail, it will be helpful toprovide background information relating to an area of statistics knownas approximate entropy ("ApEn").

I. Introduction to Approximate Entropy

More specifically, "entropy" is an area of statistics which measuresdata distributions. While moment statistics, such as mean and standarddeviation, provides useful information relating to series of data, itcannot compare and distinguish between two series containing identicaldata but whose ordering of the data, or randomness, are different.Entropy statistics provides information about the randomness of aseries, and enables one to distinguish between the randomness of twoseries. It can identify the likelihood that runs of patterns of datathat are relatively close will remain relatively close on the nextincremental comparisons. Entropy statistics, however, has not found widepractical utility and has been generally confined to the province oftheoretical mathematicians for two reasons. First, entropy hassuccessfully characterized only what is called the deterministic chaoticmodel, defined as aperiodic, seemingly random behavior in a boundedsystem. Such a system exhibits sensitive dependence on initialconditions, as opposed to the stochastic, or truly random, model, whichis representative of many real life conditions. Second, using theentropy model, an enormous series of data is generally necessary tocertify complex, deterministic settings. That is, the amount of datatypically needed to achieve convergence is impractically large for realtime applications, requiring supercomputers and/or huge amounts of timewith which to calculate the data.

More recently, an approximate entropy algorithm has been developed inorder to address the above-described practical disadvantages. Thisalgorithm is capable of quantifying the amount of regularity in chaoticor stochastic (random) systems for practical purposes. Inherent in suchan algorithm is the finding that classical entropy statistics can bemodified and, with a relatively small number of data points in a timeseries, can provide a useful summary (approximation) of the regularityof a time series represented by a single real number. An ApEn score of 0represents a totally periodic series, where there is no randomness; thetime series is 100% predictable. At the other end of the scale, an ApEnscore of 2 represents a completely stochastic, or random, andunpredictable, time series.

Following is the ApEn algorithm:

STEP 1. Form a time series of data u(1), u(2), . . . u(N). These are Nraw data values from measurements equally spaced in time.

STEP 2. Fix m, an integer, and r, a positive real number. The value of mrepresents the length of compared runs of data, and r specifies afiltering level.

STEP 3. Form a sequence of vectors x(1), x(2), . . . , x(N-m +1) inR^(m), real m-dimensional space, defined by x(i)= u(i), . . . ,u(i+m-1)!.

STEP 4. Use the sequence x(1), x(2), . . . , x(N-m+1) to construct, foreach i, 1≦i≦N-m+1, C_(i) ^(m) (r)=(number of x(j) such that dx(i),x(j)!≦r)/(N-m+1).

We must define d (x(i),x(j)! for vectors x(i) and x(j). Modify theformula by defining ##EQU1## where the u(a) are the m scalar componentsof x. d represents the distance between the vectors x(i) and x(j), givenby the maximum difference in their respective scalar components.

STEP 5. Next, define ##EQU2## where ln is the natural logarithm.

Through STEP 5, the classical entropy and ApEn algorithms are identical.The next step distinguishes between classical entropy and ApEn.

STEP 6. ApEn is defined by the equation:

    ApEn (m, r, N)=φ.sup.m (r)-φ.sup.m+1 (r).

The ApEn formula is simple to apply and requires a relatively smallamount of input data to obtain meaningful results. As the equationshows, the ApEn formula requires that three input parameters, N, m and rbe set, where: N is the total number of data points to be evaluated inthe time series; m is a positive integer representing the length of thecompared runs within the time series (i.e. vector length), and r is apositive real number representing a noise filter. The input data forApEn is a scalar time series, having typically between N=50 to 1000 datapoints. The mathematical derivation of the approximate entropy formulais further discussed in "Approximate Entropy as a Measure of SystemComplexity," Pincus, S. M., Proc. Natl. Acad. Sci. U.S.A., 88:2297-2301, 1991 and is incorporated by reference.

II. Defibrillation Apparatus And Method Employing The ApproximateEntropy Algorithm

The present invention resides in a defibrillator apparatus and relatedmethod that defibrillates a heart based on the application of an ApEnalgorithm that regulates the moment at which a regulated energy level ofelectric shock is delivered to the heart. The present inventionadvantageously identifies the moment at which the defibrillationthreshold, and thus, the required energy, is at a minimum. As such, therequired level of energy to accomplish defibrillation tends to bereduced.

With reference now to the exemplary drawings, and particularly to FIG.3a, there is shown a defibrillator 103 having a controller 104 andenergy delivery system ("EDS") 106 that is interconnected to a patient'sheart 100 via sensors 102 and therapy leads 107 and are situated on thesurface of the heart or within the heart. Any suitable defibrillatingsensors may be employed. Alternatively, a single sensor having multiplesensing poles may be employed. The sensors, or poles of a single sensor,are further advantageously deployed in a quantity sufficient to detectand transmit any appropriate amount of data indicative of the heartrhythm. To that end, they preferably, but not necessarily, have multipleelectrodes associated with the heart and transmit data in parallel tothe controller 104 contained within the defibrillator 103. Thecontroller activates the EDS also contained within the defibrillatorwhich then delivers a defibrillating shock to the heart via therapyleads 107.

As shown in FIG. 3b, the controller 104 includes a processor 108 and astorage unit 109, which together function to selectively activate theenergy delivery system 106 in response to an approximate entropy scorederived by the processor 108 from the data. More particularly, theprocessor 108 includes a fibrillation detector 110 that detects, fromthe data provided by the sensors 102, the presence of fibrillation. Anyconventional method of fibrillation detection, such as that shown instep 202 of FIG. 4, and described below, may be employed. The processoralso includes a data manipulator 114 which processes the in-parallelinput data from the sensors into a single number. The manipulation shownis an averaging function but other functions may also be used. Theprocessor further includes an ApEn computer 120, an ApEn comparator 124and an EDS activator 126. The storage unit 109 includes a data register112 which receives the in-parallel data provided by the sensors 102.This may be a parallel-in, parallel-out register or other suitabletemporary data storage component. The unit also contains a time seriesregister 116 for temporarily storing a series of data representative ofthe fibrillating heart over an equally-spaced time period. The storageunit 109 further contains an ApEn algorithm-storing memory 118 and anApEn threshold value memory 122. These memories can be read onlymemories ("ROM's") or any of a variety of conventional programmableROM's, such as EEPROM's. It will be appreciated that the programming ofsoftware, such as the Pincus ApEn algorithm, into a memory such as theApEn algorithm memory 118 is performed in a well understood manner.

In operation, when the fibrillation detector 110 detects the presence ofa fibrillation, it enables the data manipulator 114 to receive thein-parallel data from the data register 112. The manipulator 114averages the data and outputs a single real number representative of thestate of the heart at a given instant. This number is then temporarilystored in a time series register 116. The time series register has thecapacity to store at least N addressable numbers, the total number ofdata points in the time series. These steps are repeated at equal timeintervals. This creates a scalar time series of data from 1 to N, storedin the time series register, representing the state of the heart overthe time series. The ApEn computer 120 receives the time series datafrom the register 116 and applies the algorithm, or program, supplied bythe memory 118 to the scalar time series of data to compute anapproximate entropy score. Next, the comparator 124 compares thecomputed approximate entropy score of the time series to a thresholdapproximate entropy value, or range of values, stored in the ApEnthreshold value memory 122. If the proper condition, or predeterminedrelation, is met, as described in detail below and in FIG. 4, the EDSactivator 126 will signal the EDS to deliver a defibrillating shock tothe heart.

The EDS 106 includes well known types of components used in existingshock generators; namely, a capacitor, battery, and pulse generatingcircuitry (not shown). Nevertheless, in accordance with the invention,the EDS 106 uses a smaller capacitor and smaller battery, therebyreducing the weight of the defibrillator and tending to extend itsuseful life. In particular, the defibrillator of the present inventionneeds only to provide a smaller fraction of the energy that wouldotherwise be required to defibrillate a given patient. It thus uses acapacitor relatively, significantly smaller than capacitors designed inconventional defibrillators. This tends to result in two importantadvantages for ICD's: (1) the lower energy requirement translates into areduced power drain on the charging batteries, resulting in the abilityto design in smaller batteries to accomplish the same charging job asbefore. Moreover, batteries of a size presently used in defibrillatorstend to enjoy a longer life, thereby extending the usable life of theICD; and (2) the smaller EDS 106 having the smaller capacitor andbattery results in a significantly smaller control box and an attendantdecrease in discomfort for the patient within whom it is implanted.

The above advantages also foster other advantages, namely a reduction inthe shock energy level that would otherwise be necessary and anattendant lessening of the pain and discomfort that would otherwise beexperienced.

FIG. 4, in flow chart form, depicts steps taken of a method ofdefibrillation in accordance with the invention, which is particularlyapplicable to ventricular or atrial defibrillation. In an initial step200, the ventricular or atrial rate of the heart is sensed. Step 202determines whether an abnormal heart condition is present; i.e. whethera fibrillation wave is sensed. This is conventionally accomplished bydetermining whether the ventricular or atrial rate exceeds a presetvalue, x, such as 240 beats per minute ("bpm"). This value is typicallytailored to the patient, as is well known in the art. If an abnormalcondition is not sensed, the device loops back to step 200 to repeatedlysense for the presence of an abnormal condition.

When an abnormal condition is sensed, two steps occur. In step 204, theEDS is checked to determine whether it is charged and ready to deliverya shock when called upon. If it is not, the EDS is charged in step 206and enters into a standby mode, depicted by step 208. Concurrently, theon-line ApEn routine 209 commences. Specifically, in step 210, the lastN temporal data points, derived from the sensed data, are registered andcomprise the first time series of data, as described above in thedetailed description of FIG. 3b. In the preferred embodiment, the timeseries is comprised of 50 data points equally spaced in time, i.e. N=50.A time series of 50 data points is an experimentally determined quantityof data that achieves a fair balance of reliability and speed. On theone hand, 50 data points has been determined to be sufficient forreliable output from the ApEn calculation, which is the key ingredientto effective and safe low-energy defibrillation. On the other hand, a 50point series of data is small enough to facilitate fast, on-lineprocessing with state-of the-art processors. Speed, of course, isinherently important in the art of defibrillation, and particularly inventricular defibrillation, as the difference between death from cardiacarrest and life is often a matter of minutes. It is understood that N isnot limited to this number for effective ApEn defibrillation and willlikely change with the ever-increasing computational speeds ofprocessors.

In the subsequent step 212, the ApEn of the first time series iscomputed according to the ApEn algorithm. It is understood that this isbut one expression of the ApEn algorithm as applied to defibrillation,and that implementation of algorithms is performed in a well knownmanner. In one preferred embodiment, the computational speed of the ApEnof each time series is approximately 0.5 seconds.

In the next conditional step 214, the ApEn of the first time series iscompared to at least one preset ApEn threshold value. The EDS isactivated when the ApEn score has a predetermined relation with respectto the preset ApEn threshold value or values. In one preferredembodiment, the ApEn threshold value is 0.3. This number is based uponApEn input variables of vector, m=2, filter value, r=25% of the standarddeviation of the input values, and a time series having N=50 inputvalues. If the computed ApEn score is greater than the threshold value,then the pattern of fibrillation is not stable (regular) enough forsuccessful low-level defibrillation and the process repeats itself for asecond time series of data with a return to step 210. This routine isrepeated until the computed ApEn value is less than the ApEn thresholdvalue. When this occurs, the optimum condition for low leveldefibrillation is met and the charged EDS is activated in step 216 anddelivers a low level shock to the heart.

In another preferred embodiment, step 214 is modified so that thecomputed ApEn value is compared to two ApEn threshold values--a lowerthreshold and an upper threshold. In this embodiment, if the ApEn valueis either below the lower threshold or above the upper threshold the EDSis activated to provide a low level defibrillating shock.

After the shock is delivered, the process returns to step 202 forevaluation. Thus, if the defibrillating shock was successful and normalsinus rhythm is returned, step 202 responds negatively to the abnormalcondition inquiry, and the step 200-to step 202-back to step 200 loopwill continue indefinitely, until the next episode of fibrillation issensed. However, if the shock was not successful in converting the heartto normal sinus rhythm, then the ApEn routine 209 and the EDS chargingroutine, steps 204 to 208, will once again be invoked. This entireprocess will continue until one of the following conditions are met: (a)a low level shock finally succeeds in defibrillating the patient; (b) apreset limit to the number of shocks delivered is reached (not shown);or (c) a preset duration of time delay since the onset of fibrillationis reached. If either of the last two conditions occur, thedefibrillator is instructed to revert to a conventional defibrillationmode. It is appreciated that the preset limit or duration for atrialdefibrillation may be larger than for ventricular defibrillation becauseAF is not immediately life threatening as is VF.

Underlying the present invention is the understanding that theconventional assumption that the fibrillation mechanism is totallyrandom is incorrect. Using detailed computer mapping techniques to trackthe activity of fibrillating hearts we discovered that the fibrillationparadigm is not totally random but rather exhibits chaotic behavior. Infact, there exist patterns of regularity in the "abnormal" activationpatterns of a fibrillating heart.

Moreover, it is evident from the foregoing that analysis of thesepatterns of fibrillation can be valuable in treating this potentiallydeadly condition. Specifically, at moments of relative regularity ofheart activation during a fibrillation episode (low approximate entropyscore), the fibrillation threshold is reduced. That is, converting adefibrillating heart to normal sinus rhythm can be achieved with theapplication of an electric shock having an energy level significantlylower than previously possible. The same phenomenon (reduction in thedefibrillation threshold) has been found to occur when the electricalactivity of the heart displays a relatively high degree of randomness(high approximate entropy score).

The ApEn algorithm enables identification of these moments of relativeregularity or relative irregularity. Specifically, the moment a chaoticfibrillation episode manifests a relatively low or high ApEn value thedefibrillation threshold is significantly reduced. An example of anexperiment that was conducted to apply the theory is detailed below. Itwill be understood, however, that the particular tests, including thenumber of electrodes mentioned, are set forth for the purpose ofproviding an example to amplify the foregoing description of theinvention.

1. Testing

VF was induced in swine hearts by applying bi-phasic electrical shocksacross the myocardium. 500 electrodes were placed on the heart tissue tosense and record electrical activity. This activity was repeatedlyrecorded as 500 distinct channels using computerized activation mappingtechniques at short intervals. At various moments during thefibrillation episodes, defibrillating shocks of varying intensities wereapplied.

2. Results

Table 1 shows the results of one series of such tests:

                  TABLE 1                                                         ______________________________________                                                                       Defibrillation                                 Test Number                                                                             ApEn      Energy Applied                                                                           Outcome                                        ______________________________________                                        1         0.446     2.0 joules Failure                                                  0.271     1.7 joules Success                                        2         0.470     1.5 joules Failure                                                  0.274     1.4 joules Success                                        3         0.249     1.3 joules Failure                                                  0.236     1.3 joules Success                                        4         0.616     1.2 joules Success                                                  0.311     1.3 joules Failure                                        ______________________________________                                    

In test numbers 1 through 3 defibrillation was successful only when theApEn was lower than 0.3. In test 4 defibrillation was successful whenthe ApEn was above 0.6. Additional testing has supported these findings.

It will be observed that the present invention also has application toexternal defibrillators. Among other things, it tends to relatively,significantly reduce the shock energy otherwise needed for successfulconversion and to lessen the severe pain historically associated withthe defibrillation shock.

From the foregoing description, it should be apparent that the presentinvention provides a method and apparatus for low energy defibrillationwhich times the delivery of the shock to the fibrillating heart tocoincide with the moment that the defibrillation threshold is at aminimum.

Although the invention has been described in detail with reference onlyto the presently preferred devices and method, those of ordinary skillin the art will appreciate that various modifications can be madewithout departing from the invention. Accordingly, the invention isdefined only by the following claims.

We claim:
 1. An apparatus for defibrillating a heart, comprising:a firstsensor associated with the heart, the sensor being configured to detecta heart rhythm and transmit data indicative of the rhythm; an energydelivery system associated with the heart for delivery to the heart anelectric shock of a predetermined energy level; and a controllerresponsive to the sensor that selectively activates the energy deliverysystem in response to an approximate entropy score derived by thecontroller from the data.
 2. The apparatus of claim 1, wherein theenergy delivery system comprises:an electric shock generator; and atleast one therapy electrode connected to the electric shock generator.3. The apparatus of claim 2, wherein the apparatus is an externaldefibrillator.
 4. The apparatus of claim 2, wherein the apparatus is animplantable defibrillator.
 5. The apparatus of claim 4, wherein thefirst sensor is associated with a ventricle of the heart.
 6. Theapparatus of claim 4, wherein the first sensor is associated with anatrium of the heart.
 7. The apparatus of claim 4, wherein the firstsensor and therapy electrode are integral.
 8. The apparatus of claim 4,wherein the first sensor includes multiple sensing poles.
 9. Theapparatus of claim 1, further including a second sensor associated withthe heart and being configured to detect a heart rhythm and transmitdata indicative of the rhythm.
 10. The apparatus of claim 1, wherein thecontroller is adapted to repeatedly compute from the data substantiallyequally spaced in time discrete data points comprising a scalar timeseries.
 11. The apparatus of claim 10, wherein the controller derivesthe approximate entropy score from the scalar time series.
 12. Theapparatus of claim 11 wherein the approximate entropy score is definedby the equation:

    ApEn(m,r,N)=φ.sup.m (r)-φ.sup.m+1 (r).


13. The apparatus of claim 12 wherein the approximate entropy scoreranges from 0 to
 2. 14. The apparatus of claim 13 wherein the controlleris adapted to: (a) compare the approximate entropy score to a firstpreset approximate entropy threshold value stored in a memory of thecontroller; and (b)selectively activate the energy delivery system whenthe approximate entropy score has a predetermined relation with respectto the first preset approximate entropy threshold value.
 15. Theapparatus of claim 14 wherein the predetermined relation arises when theapproximate entropy score is less than the first preset approximateentropy threshold value.
 16. The apparatus of claim 15 wherein the firstpreset approximate entropy threshold value is approximately 0.3 whenN=500, m=2 and r=approximately 25% of the standard deviation of thescalar time series.
 17. The apparatus of claim 13 wherein the controlleris adapted to: (a) compare the approximate entropy score to a firstpreset approximate entropy threshold value and a second presetapproximate entropy threshold value stored in a memory of thecontroller; and (b)selectively activate the energy delivery system whenthe approximate entropy score is either less than the first presetapproximate entropy threshold value or greater than the second presetapproximate entropy threshold value.
 18. The apparatus of claim 1wherein the predetermined energy level is substantially less than about10 joules.
 19. The apparatus of claim 18 wherein the preset energy levelis substantially less than about 5 joules.
 20. A defibrillator for aheart, comprising:a sensor associated with the heart, the sensor beingconfigured to detect a heart arrhythmia and transmit data indicative ofthe arrhythmia; an energy delivery system associated with the heart fordelivery to the heart an electric shock of a predetermined energy level;and a controller including;a sensor data processor receptive of the datato repeatedly compute a plurality of data points substantially equallyspaced in time; an approximate entropy processor to repeatedly computean approximate entropy score from the data points; a comparator tocompare the approximate entropy score to at least one preset approximateentropy threshold value stored in a memory of the controller; and anenergy delivery system activator to monitor the output of the comparatorand to activate the energy delivery system when the approximate entropyscore has at least one predetermined relation with respect to the atleast one preset approximate entropy threshold value, therebydefibrillating the heart.
 21. A defibrillator for a heart, comprising:anenergy delivery system associated with the heart that delivers to theheart an electric shock of a predetermined energy level; and acontroller that receives data indicative of the rhythm of the heart andthat selectively activates the energy delivery system in response to anapproximate entropy score derived by the controller from the data. 22.The defibrillator of claim 21, wherein the energy delivery systemcomprises:an electric shock generator; and at least one therapyelectrode connected to the electric shock generator.
 23. Thedefibrillator of claim 22, wherein the defibrillator is external. 24.The defibrillator of claim 22, wherein the defibrillator is implanted.25. The defibrillator of claim 21, wherein the controller: (a) isadapted to repeatedly compute from the data substantially equally spacedin time data points comprising a scalar time series; and (b) derives theapproximate entropy score from the scalar time series.
 26. Thedefibrillator of claim 25, wherein the approximate entropy score isdefined by the equation:

    ApEn(m,r,N)=φ.sup.m (r)-φ.sup.m+1 (r).


27. The defibrillator of claim 26, wherein the approximate entropy scoreranges from 0 to
 2. 28. The defibrillator of claim 27, wherein thecontroller is adapted to: (a) compare the approximate entropy score toat least one preset approximate entropy threshold value stored in amemory of the controller; and (b) selectively activate the energydelivery system when the approximate entropy score has a predeterminedrelation with respect to the at least one preset approximate entropythreshold value.
 29. The defibrillator of claim 21, wherein the presetenergy level is substantially less than about 10 joules.
 30. Thedefibrillator of claim 29, wherein the preset energy level issubstantially less than about 5 joules.
 31. A defibrillator for a heart,comprising:means for sensing an arrhythmia and for generating dataindicative of the arrhythmia; means associated with the heart forproviding at least one electric shock to the heart; and means forselectively activating the means for providing the at least one electricshock based upon an approximate entropy score.
 32. The defibrillator ofclaim 31, wherein the activating means further includes means fordetermining the approximate entropy score based on the data.
 33. Thedefibrillator of claim 32, wherein the selectively activating meansfurther includes means for comparing the approximate entropy score to atleast one preset approximate entropy threshold value and for activatingthe shock providing means when the approximate entropy score has apredetermined relation with respect to the at least one presetapproximate entropy threshold value.
 34. A method for defibrillating aheart, comprising:sensing a heart arrhythmia and generating dataindicative of the arrhythmia; determining an approximate entropy scorebased on the data; selectively activating an energy delivery system todeliver a defibrillating shock to the heart based on the approximateentropy score.
 35. The method as defined in claim 34, wherein the methodfurther includes comparing the approximate entropy score to at least onepreset approximate entropy threshold value.
 36. The method as defined inclaim 35, wherein the energy delivery system is activated when theapproximate entropy score has a predetermined relation with respect tothe preset approximate entropy threshold value.
 37. The method asdefined in claim 34, wherein the heart arrhythmia sensed is ventricularfibrillation.
 38. The method as defined in claim 34, wherein the heartarrhythmia sensed is atrial fibrillation.